A diode-pumped, solid state laser has a higher operating efficiency than a flashlamp-pumped, solid state laser, due to the good spectral matching of the light emitted by a diode to the absorptive region of the solid state laser. The wavelength emitted by the laser diode is extremely temperature-sensitive and therefore requires a temperature control mechanism to stabilize the diode at a set operating temperature. The control mechanism can be achieved by use of thermoelectric coolers, liquid cold plates, or air-cooled heat exchangers. A dichotomy exists as to the function of the heat transfer mechanism. At high ambient temperatures, the heat transfer mechanism of the system must enable low thermal resistance from the diode to the ambient in order to ensure that the temperature of the diode does not rise above the setpoint temperature. At low ambient temperatures in the "stand-by" mode, the diode laser is heated in order to stabilize the laser at the setpoint temperature. Therefore, at low temperatures, the thermal resistance to the ambient must be large, in order to decrease heat losses to the environment. This dichotomy is the reason for the development of a thermal switch, or thermal "clutch," which enables a low thermal resistance to the ambient at high ambient temperatures and a high thermal resistance at low ambient temperatures.
U.S. Pat. No. 4,515,206 (Carr) describes a generalized heat switch on the basis of a liquid crystal (LC) cell, with no particular application stated. The Carr design of the LC cell includes LC material encapsulated within a Teflon cell, with electrodes placed at both ends of the cell. High voltage (DC or AC) applied to the cell induces electrohydrodynamic (EHD) motion, and thus increases the effective thermal conductivity of the LC within the cell.
EHD motion is formed due to a dipole, or moment, applied to LC molecules when they are placed within an electric field. In order for there to be a moment on the molecules, they have to be initially oriented in a plane parallel to the electrode surface. It is stated in the Carr patent that molecular alignment of an LC is achieved by the wall effect of the electrode, which causes the molecules to align in the parallel direction. In order to ensure that the molecules will be correctly aligned throughout the bulk of this prior art device, it is suggested to apply a magnetic field in the direction parallel to the plane of the electrodes. Subsequent application of an electric field will result in EHD motion.
U.S. Pat. No. 5,222,548 (Biggers et al.) describes a heat valve having a concept similar to that of Carr, with a number of changes. The heat valve of the Biggers patent is used in divers' wetsuits in order to regulate the heat transfer from the diver to the water to avoid hypo/hyperthermia. The Biggers concept is based on applied AC voltage alone, with an optimum voltage and frequency needed for each cell geometry, such as cell cross-sectional area, the distance between electrodes, etc.
The Biggers design uses AC voltages to induce EHD motion. Use of AC voltages has its advantages, but also has disadvantages. An LC cell based on AC voltage design requires the optimization of frequency and voltage for each cell geometry, a procedure which is timely and costly. The use of AC voltages results in EMI/RFI noise, which can interfere with the diode laser operation. In addition, the power supplies for AC voltages at varied frequencies are more costly and cumbersome than DC power supplies.
Other types of heat switches are known, but all suffer from various disadvantages, including moving parts, high power consumption, large volume and gravity-dependence.